In a communication network, the construction cost and maintenance cost of a private network is very high, the construction period is very long, and the private network doesn't have universality. Therefore, construction of a private network at present generally uses an overlay network mode, i.e., special devices satisfying a private requirement are overlaid on an universal communication network to implement functions of the private network. For example, an encrypted communication private network may be constructed on the basis of a CDMA (Code Division Multiple Access) communication network, and the CDMA network only implements transmission of encrypted voice and doesn't implement decryption of the encrypted voice.
In the communication field, with the evolution of the 3G (Third Generation) mobile communication, the architecture of a CDMA mobile communication network is divided into two parts: a radio access network and a core network.
As shown in FIG. 1, the MSC (Mobile Switching Center) has evolved into two parts: a MSCe (Mobile Switching Center Emulation) 101 and a MGW (Media Gate Way) 103, so that controlling and bearing are separated, and the TDM (Time Division Multiplex) circuit switching technology is replaced with the packet network technology. Wherein, the MSCe 101 provides the function of call control and mobility management, the MGW 103 provides the function of media control, provides transmission resource and has the function of media stream manipulation. The MSCe 101 interchanges a signaling stream 105 with the base station 102, MGW 103 and tandem exchange 104, and a media stream 106 is interchanged among the base station 102, MGW 103 and tandem exchange 104.
The MSCe 101 and MGW 103 are two network elements belonging to the core network, wherein the MGW103 implements conversion between different encoding-decoding formats of different packet domains (i.e. IP domains) or those of a packet domain and a circuit domain (i.e. TDM domain). Such conversion of encoding-decoding format is referred to as RTO (Remote Transcode Operation), and during the RTO, the compression, encoding and decoding of a media stream will cause impairment of the media stream. If the encoding-decoding formats of both ends in a packet domain are the same, then conversion of encoding-decoding format is not needed, which is referred to as TrFO (Transcoder Free Operation), and during the TrFO, the compression, encoding and decoding of a media stream will not cause impairment of the media stream.
An encrypted call from a private terminal to another, i.e. the end-to-end encrypted call, is referred to as a cipher-call; accordingly, the end-to-end non-encrypted call is referred to as a plain-call. A User may initiate a call by a plain-call, and when conversation begins, press a button for switching from a plain-call to a cipher-call on a private terminal to switch the plain-call to a cipher-call, and this process is referred to as plain-call to cipher-call switch.
In a traditional circuit domain, all media streams are borne in the circuit domain, without being encoded or decoded, so there is no operation that will impair the media streams. However, between different packet domains or in a 3G network with packet domain and circuit domain hybrid networking, if RTO of a media stream is performed after entering the cipher-call stage, the compression, encoding and decoding of the media stream will cause impairment of the media stream, and thus reliability of the transmission will be affected.
For example, if transparent lossless transmission of encrypted voice information is implemented within a CDMA network, the encrypted voice information is not decrypted to achieve reliable transmission of the encrypted communication. After a private terminal switches from a plain-call to a cipher-call, if RTO of the media stream is needed, the compression, encoding and decoding of the media stream will cause impairment of the media stream, and thus greatly affect the transmission reliability.
From the discussion above it can seen that the existing technology of media stream transmission after switching from a plain-call to a cipher-call has shortcomings, and has inconvenience and defects in actual use, thus needs to be improved.